This invention relates to a strap that may be comfortably worn on the arm or leg, that has two or more sections of conductive material for improved wicking of static electrical charges from the wearer's body and that has or is connected to circuitry that activates audible or visual indicators if certain failure conditions exist.
Static electricity causes problems in the electronics and other industries, particularly with the advent of integrated circuits and other microelectronic components. Components such as integrated circuits, for example, may be disabled or destroyed by over-voltages or power density resulting from static electricity. Certain junctions in such circuits can be destroyed by as little as a 50-volt potential, which radically changes the doping structure in their lattices. Power densities resulting from excessive potential and imperfections in circuit layout or structure can vaporize or radically alter the silicon substrate and thus impair or destroy a circuit's performance. Yet a person walking on carpet on a dry day can accumulate as much as 30,000 volts of potential, and he can triboelectrically generate thousands of volts by simply changing his position in his chair or handling a styrofoam cup.
Such a person can inadvertently discharge such static potential into a circuit or component by touching it and causing over-voltage or excessive power density. Additionally, the potential in such a person's body can induce a charge in a circuit that can later cause over-voltage or excessive power density when the circuit is subsequently grounded.
More and more frequently, therefore, those in industries in which integrated circuits and other microelectric components are handled or assembled are taking measures to limit the failure rate of those circuits and components by attempting to keep them as well as their environment at zero electrical potential. Such measures include providing workers and work stations with anti-static carpet, conductive or dissipative grounded desk top work surfaces, hot air ion generators which emit ions to neutralize static charges, and grounding straps to keep workers at zero potential.
The term "conductive" herein, and according to its customary usage in the art, means an electrical resistence of between 0 and 10.sup.5 ohms. Similarly, "dissipative" means a resistence of between 10.sup.5 and 10.sup.9 ohms, "anti-static" means a resistence of between 10.sup.9 and 10.sup.14 ohms and "insulative" means a resistence of more than 10.sup.14 ohms.
The situations in which grounding straps are used heighten the importance of their being reliable in maintaining continuous electrical contact with the skin. The person working on microelectronic components for integrated circuits may be completely unaware that he has accumulated minor static electrical charges, and may therefore unknowingly be in a position to disable circuits on which he is working or which he is handling. If his strap is lose or he has removed it, he may be unaware that electrical discharges transmitted from his fingers are disabling the circuits. (A typical person cannot sense a static electrical discharge of less than approximately 3,500 volts.) No one may discover that the circuits have been disabled or damaged until hours, days or weeks later, when the circuits have been placed in components or devices which fail in the field. Removal and repair or replacement of the circuits once in the field is far costlier than avoiding potential failure while the wearer is handling the circuits. Thus, the wearer's employer typically must depend upon the effectiveness of the wrist strap to maintain a lower failure rate of such electronic circuits and components by ensuring that the strap maintains continuous electrical contact with the wearer's wrist.
These considerations have been addressed by several types of grounding straps. U.S. Pat. No. 4,373,175 issued Feb. 8, 1983 to Mykkanen, for instance, discloses an extensible metal band similar to a "Speidel" watch band on which a snap fastener for a grounding cord is attached. Such a strap includes only one contact and line connecting the strap with ground, however, and its conductive metal outer surface can prove dangerous to the wearer if it contacts an electrical potential sufficient to electrocute the wearer. Further, the Mykannen strap includes no visual or aural means to inform the wearer or others when the strap loses electrical connection with ground.
Another grounding strap is disclosed in U.S. Pat. No. 3,857,397 issued Dec. 31, 1984 to Brosseau. Outer and inner conductive polyolefin layers sandwich an intermediate nylon scrim layer to form the band. Hook and loop (Velcro) fastening material holds the strap on the wrist. This strip is typical of a number of straps having carbon-suffused synthetics or other conductive polyolefins. Body oils and minerals can accumulate on such surfaces and interfere with electrical contact between the band and the skin. Further, the Brosseau strap includes only one electrical contact surface and grounding, and that patent discloses no visual or aural means to inform the the wearer or others of failure conditions.
Another approach to many of these problems is disclosed in U.S. Pat. No. 4,398,277 issued Aug. 9, 1983 to Christiansen and Westberg. This strap is made of knitted stretchable fabric containing stainless steel fibers. A plastic and metal fitting permanently closes the strap into a loop of predetermined size and has a connection for a grounding cord. As in the case of the straps mentioned above, the Christiansen strap includes only one electrical contact surface and grounding line, and no visual or aural means is disclosed for informing the wearer of failure conditions.